<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>本实例中参数a将返回undefined</title>
    <script type="text/javascript">
        function getValue() {
            var a, b, c;
            a = void (b = 5, c = 7);
            document.write('a = ' + a + ' b = ' + b + ' c = ' + c);
        }
    </script>
</head>

<body>
    <p>点击以下按钮查看结果：</p>
    <form>
        <input type="button" value="点我" onclick="getValue();" />
    </form>
</body>

</html>